The invention relates to new composite glass/plastic material ophthalmic lenses with a high index of refraction and their manufacturing.
Composite lenses are desirable products to the extent that the part made of plastic material contributes lightness of weight and shock resistance while the glass part provides resistance to radiation, ability to receive a wide range of lasting surface treatments (anti-reflecting, reflecting, coloration) and, if necessary, lasting photochromism.
Through European Patent Application No. 182,503 [which corresponds to U.S. Patent No. 4,679,918 (Ace)] for example, composite lenses are known which consist of a lens made of plastic material stuck to a glass element of constant thickness by means of a relatively thick elastomer adhesive layer. The disadvantage of such lenses is their relatively heavy weight and especially their excessive thickness, particularly at the edge in the case of negative power lenses, which makes them aesthetically not very attractive.
Attempts have also been made to produce composite lenses by casting a liquid thermosetting plastic material onto a thin glass element, followed by heating with the intention of hardening the plastic material. This process has not been successful because of the great stresses exerted on the glass element by the plastic part during cooling because of the extensive contraction of the plastic material, stresses which cause the glass element to break or which bring about other redhibitory defects, as is explained in the preamble of French Patent No. 2,407,898 [which corresponds to U.S. Patent No. 4,227,950 (Spycher)].
As a solution to the problems encountered in prior techniques, French Patent No. 2,407,898 proposes a process of direct pouring for the production of a composite article which consists of a glass element connected to a thermosetting plastic material which exhibits a high degree of contraction, the composite article demonstrating low residual stresses between the glass element and the plastic element, which comprises the operations consisting of covering selected portions of the surface of the glass element with a thermoplastic adhesive with a hot gluing temperature higher than the minimum hardening temperature of the thermosetting plastic material; of pouring the thermosetting plastic material in liquid form over the portions of the surface of the glass element which were covered with the thermoplastic adhesive; of curing the thermosetting plastic material by heating the plastic material, the glass element, and the thermoplastic adhesive to a temperature lower than the hot gluing temperature of the thermoplastic adhesive, but higher than the minimum temperature of hardening of the thermosetting material; and of solidly connecting the cured thermosetting plastic material, the thermoplastic adhesive, and the glass element into a single glass-plastic material composite article by heating these components to a temperature higher than the hot gluing temperature of the thermoplastic adhesive.
Composite lenses produced by this process have, however, not been developed industrially because of their complicated manufacturing process.
There is, therefore, an unsatisfied need for composite lenses which are lightweight, not very thick, and which are easy and economical to manufacture.